Services are typically requested and deliverables typically returned in the form of messages between computers (or between software applications running on these computers).
For example, messaging is commonly used to request services (such as micro-services) in scalable and distributed Information Technology (IT) systems because of the way the messaging can provide a buffer or “shock-absorber” between the requesting application and the service. If the service is busy processing data from another application for example, new requests can be queued until an instance of the service becomes available.
To cater for the large number of applications an organization might have running at one time (and thus the associated large workload), it is common for a service to be scaled to multiple instances.